The mineral tetrahedrite Cu12Sb4S13 exhibits a first-order metal-insulator transition (MIT) at TMI = 85 K and ambient pressure. We measured the 63 Cu-NMR at ambient pressure and the resistivity and magnetic susceptibility at high pressures. 63 Cu-NMR results indicate a nonmagnetic insulating ground state in this compound. The MIT is monotonically suppressed by pressure and disappears at ∼ 1.0 GPa. Two other anomalies are observed in the resistivity measurements, and the pressure -temperature phase diagram of Cu12Sb4S13 is constructed.Mixed-valent transition-metal (TM) compounds with a fractional number of electrons per TM site often exhibit a metal-insulator transition (MIT) associated with charge ordering. The insulating ground state is either nonmagnetic or magnetically ordered depending on the compound. Typical compounds with nonmagnetic ground states include Magneli phase Ti 4 O 7 with Ti 3.5+ (3d 0.5 ), 1) oxide spinel AlV 2 O 4 with V 2.5+ (3d 2.5 ), 2) and thiospinel CuIr 2 S 4 with Ir 3.5+ (5d 5.5 ). 3) For instance, CuIr 2 S 4 experiences an MIT at 230 K associated with the charge ordering of the mixed-valent Ir ion. The formation of isomorphic octamers of Ir 3+ 8 S 24 and Ir 4+ 8 S 24 with the dimerization of Ir 4+ (5d 5 , S = 1/2) pairs results in a spin-singlet ground state. 3, 4) On the other hand, vanadium bronze β-Na 0.33 V 2 O 5 with (1/6)V 4+ + (5/6)V 5+ (3d 1/6 ), 5) chromium hollandite K 2 Cr 8 O 16 with Cr 3.75+ (3d 2.25 ), 6) and magnetite Fe 3 O 4 with Fe 2.5+ (3d 5.5 ) for a B sublattice 7, 8) exhibit magnetically ordered ground states. Interestingly, superconductivity appears in β-Na 0.33 V 2 O 5 5) and CuIr 2 S 4 9, 10) when charge ordering is suppressed by applying hydrostatic pressure and chemical doping, respectively.The mineral tetrahedrite Cu 12 Sb 4 S 13 is another example of a mixed-valent TM compound and has been attracting considerable interest because of its enhanced thermoelectric properties. 11-15) Apart from its thermoelectricity, Cu 12 Sb 4 S 13 exhibits intriguing physical properties such as an MIT and a simultaneous sudden decrease in the magnetic susceptibility χ at 85 K, 11,16) which is reminiscent of thiospinel CuIr 2 S 4 . 9, 10) However, understanding this transition is not intuitive be- *